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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
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NASA NTRS1971

Cosmic rays from supernovae and comments on the Vela X pre-supernova

A possible history of the production of elements in the galaxy is presented, based on assumptions about the end points of stellar evolution and of the general evolution of the galaxy. A wide range of quantities involving the relative abundances of nucleosynthesis products observed in the solar system, and various galactic quantities such as the current rate of supernova production and the present gas content of the galaxy, were considered. These assumptions were utilized in a computer program in which the gas content of the galaxy is gradually turned into stars. The stars are continually enriched in the products of nucleosynthesis as they approach the ends of their evolutionary lifetimes. It is suggested that supernova explosions are associated with the mass range of about 4-8 solar masses. Possible theories on the type of stellar explosive event represented by the Vela supernova are discussed.

Cameron, A. G. W.↗
NASA NTRS1995

Beryllium and Boron abundances in population II stars

The scientific focus of this program was to undertake UV spectroscopic abundance analyses of extremely metal poor stars with attention to determining abundances of light elements such as beryllium and boron. The abundances are likely to reflect primordial abundances within the early galaxy and help to constrain models for early galactic nucleosynthesis. The general metal abundances of these stars are also important for understanding stellar evolution.

Source record↗
NASA NTRS1994

Exotic fluoride molecules in IRC +10216: Confirmation of AlF and searches for MgF and CaF

Three new rotational transitions of aluminum fluoride (AlF) at 0.8 and 1.2 mm have been observed. The J = 10-9, J = 8-7, and J = 7-6 lines of AlF at 230, 263, and 329 GHz, respectively, were seen toward IRC +10216 using the Caltech Submillimter Observatory (CSO). Combined with the earlier data obtained for this species at IRAM at 2 and 3 mm, these measurements confirm the presence of the metal halide in this carbon-rich circumstellar shell. Analysis of the CSO and IRAM data suggests that AlF arises from a source with a diameter of theta(sub s) approximately = 5-10 sec and hence is present chiefly in the inner envelope of IRC +10216. In this region, the molecule has a column density of (0.3-1.1) x 10(exp 15)/sq cm, which indicates a fractional abundance of at least approximately 10(exp -9), relative to H2. Searches for the metal fluoride species CaF and MgF have also been conducted toward IRC +10216, but with negative results. The column density upper limits for MgF and CaF are N(sub tot) less than (1-4) x 10(exp 14)/sq cm. Relative abundances of these metal fluoride molecules can be understood in terms of chemical thermodynamic equilibrium. The presence of AlF in IRC +10216 also indicates that large quantities of fluorine must be present in the inner stellar envelope, suggesting that this element may be produced not primarily in explosive nucleosynthesis but rather in helium shell flashes, as indicated also by HF spectroscopy of red giant stars.

Ziurys, L. M.↗
NASA NTRS1989

Primordial lithium and the standard model(s)

The results of new theoretical work on surface Li-7 and Li-6 evolution in the oldest halo stars are presented, along with a new and refined analysis of the predicted primordial Li abundance resulting from big-bang nucleosynthesis. This makes it possible to determine the constraints which can be imposed on cosmology using primordial Li and both standard big-bang and stellar-evolution models. This leads to limits on the baryon density today of 0.0044-0.025 (where the Hubble constant is 100h km/sec Mpc) and imposes limitations on alternative nucleosynthesis scenarios.

Deliyannis, Constantine P.↗
NASA NTRS1980

A first roadmap for kryptology

Studies of the complex variations of the isotopic composition of xenon in the solar system have been christened 'xenology'. In the title of the present investigation, the word 'kryptology' is employed to indicate the primary objective of the reported studies. This objective is related to the prediction of the isotopic composition of krypton which comes from a number of specific locations of a supernova in association with the isotopic compositions of xenon from these locations. Krypton is a logical candidate for testing the stellar theory on geochemical grounds, taking into account also the point of view of nucleosynthesis, because the isotopes of xenon and krypton are formed by the same thermonuclear processes in stars. The data and arguments presented in the investigation show that the treatment by Heymann and Dziczkaniec (1979), although not wrong, is too simplistic, because it has ignored the possibility of holdup and arrest in Xe network.

Heymann, D.↗
NASA NTRS1991

Gamma-ray spectroscopy: The diffuse galactic glow

The goal of this project is the development of a numerical code that provides statistical models of the sky distribution of gamma-ray lines due to the production of radioactive isotopes by ongoing Galactic nucleosynthesis. We are particularly interested in quasi-steady emission from novae, supernovae, and stellar winds, but continuum radiation and transient sources must also be considered. We have made significant progress during the first half period of this project and expect the timely completion of a code that can be applied to Oriented Scintillation Spectrometer Experiment (OSSE) Galactic plane survey data.

Hartmann, Dieter H.↗
NASA NTRS1993

The astrophysical interpretation of isotope anomalies in graphite and SiC grains of chondrites

The C, N, and Mg isotopic compositions in graphite and SiC grains of carbonaceous chondrites can be explained by nuclear processes in massive O,B stars of second generation passed a stage of WR star with intensive stellar wind, where grain condensation had taken place. The interstellar graphite and SiC grains with anomalous isotopic compositions of C, N, Ne, and Si and other elements of nucleosynthetic origin, found in non-equilibrated chondrites, are most suitable for determination of astro-physical objects, where nucleosynthesis had taken place. Two objects were examined: (1) massive O,B stars of second generation passed a stage of WR star with intensive stellar wind (O,B-WR model) and (2) low-mass stars (1 less than or equal to M/solar mass less than or equal to 3) during thermally pulsing asymptotic giant branch phase (TP-AGB model).

Lavrukhina, A. K.↗
NASA NTRS1982

Binary stars: Mass transfer and chemical composition

It is noted that mass exchange (and mass loss) within a binary system should produce observable changes in the surface chemical composition of both the mass losing and mass gaining stars as a stellar interior exposed to nucleosyntheses is uncovered. Three topics relating mass exchange and/or mass loss to nucleosynthesis are sketched: the chemical composition of Algol systems; the accretion disk of a cataclysmic variable fed by mass from a dwarf secondary star; and the hypothesis that classical Ba II giants result from mass transfer from a more evolved companion now present as a white dwarf.

Lambert, D. L.↗
NASA NTRS2004

History of the Nuclei Important for Cosmochemistry

An essential aspect of studying the nuclei important for cosmochemistry is their production in stars. Over the grant period, we have further developed the Clemson/American University of Beirut stellar evolution code. Through use of a biconjugate-gradient matrix solver, we now routinely solve l0(exp 6) x l0(exp 6) sparse matrices on our desktop computers. This has allowed us to couple nucleosynthesis and convection fully in the 1-D star, which, in turn, provides better estimates of nuclear yields when the mixing and nuclear burning timescales are comparable. We also have incorporated radiation transport into our 1-D supernova explosion code. We used the stellar evolution and explosion codes to compute iron abundances in a 25 Solar mass star and compared the results to data from RIMS.

Meyer, Bradley S.↗
NASA NTRSDate not supplied

Heavy-metal Abundances in DO-type White Dwarfs

Spectral lines of a variety of trans-iron elements were identified in high-resolution ultraviolet spectra of DO-type white dwarfs. Abundance determinations by means of non-local thermodynamic equilibrium stellar-atmosphere techniques have shown that, without exception, their abundances are unexpectedly strong supersolar, much higher than predicted by asymptotic-giant-branch nucleosynthesis calculations. We present our analyses and discuss the photospheric trans-iron element prominence.

Thomas Rauch↗
NASA NTRS1985

Gamma-ray line emission from Al-26 produced by Wolf-Rayet stars

The recent satellite observations of the 1.8 MeV line from the decay of Al-26 has given a new impetus to the study of the nucleosynthesis of Al-26. The production and ejection of Al-26 by massive mass-losing stars (Of and WR stars) is discussed in the light of recent stellar models. The longitude distribution of the Al-26 gamma ray line emission produced by the galactic collection of WR stars is derived based on various estimates of their radial distribution. This longitude profile provides: (1) a specific signature of massive stars on the background of other potential Al-26 sources, as novae, supernovae, certain red giants and possibly AGB stars; and (2) a possible tool to improve the data analysis of the HEAO 3 and SMM experiments.

Prantzos, N.↗
NASA NTRS2004

Nucleosynthesis of Short-lived Radioactivities in Massive Stars

A leading model for the source of many of the short-lived radioactivities in the early solar nebula is direct incorporation from a massive star [1]. A recent and promising incarnation of this model includes an injection mass cut, which is a boundary between the stellar ejecta that become incorporated into the solar cloud and those ejecta that do not [2-4]. This model also includes a delay time between ejection from the star and incorporation into early solar system solid bodies. While largely successful, this model requires further validation and comparison against data. Such evaluation becomes easier if we have a better sense of the nature of the synthesis of the various radioactivities in the star. That is the goal of this brief abstract.

Meyer, B. S.↗
NASA NTRS2008

Heavy Elements and Cool Stars

We report on progress in the analysis of high-resolution near-IR spectra of alpha Orionis (M2 Iab) and other cool, luminous stars. Using synthetic spectrum techniques, we search for atomic absorption lines in the stellar spectra and evaluate the available line parameter data for use in our abundance analyses. Our study concentrates on the post iron-group elements copper through zirconium as a means of investigating the slow neutron-capture process of nucleosynthesis in massive stars and the mechanisms that transport recently processed material up into the photospheric region. We discuss problems with the atomic data and model atmospheres that need to be addressed before theoretically derived elemental abundances from pre-supernova nucleosynthesis calculations can be tested by comparison with abundances determined from observations of cool, massive stars.

Wahlgren, Glenn M.↗
NASA NTRS1988

Isotopic anomalies - Chemical memory of Galactic evolution

New mechanisms for the chemical memory of isotopic anomalies are proposed which are based on the temporal change during the chemical evolution of the Galaxy of the isotopic composition of the mean ejecta from stars. Because of the differing temporal evolution of primary and secondary products of nucleosynthesis, the isotopic composition of the bulk interstellar medium changes approximately linearly with time, and thus any dust component having an age different from that of average dust will be isotopically anomalous. Special attention is given to C, O, Mg, Si, and isotopically heavy average-stellar condensates of SiC.

Clayton, Donald D.↗
NASA NTRS1991

Baryonic dark matter

Both canonical primordial nucleosynthesis constraints and large-scale structure measurements, as well as observations of the fundamental cosmological parameters, appear to be consistent with the hypothesis that the universe predominantly consists of baryonic dark matter (BDM). The arguments for BDM to consist of compact objects that are either stellar relics or substellar objects are reviewed. Several techniques for searching for halo BDM are described.

Silk, Joseph↗
NASA NTRS1988

Cosmological quantum chromodynamics, neutron diffusion, and the production of primordial heavy elements

A simple one-dimensional model is used to describe the evolution of neutron density before and during nucleosynthesis in a high-entropy bubble left over from the cosmic quark-hadron phase transition. It is shown why cosmic nucleosynthesis in such a neutron-rich environment produces a surfeit of elements heavier than lithium. Analytical and numerical techniques are used to estimate the abundances of carbon, nitrogen, and heavier elements up to Ne-22. A high-density neutron-rich region produces enough primordial N-14 to be observed in stellar atmospheres. It shown that very heavy elements may be created in a cosmological r-process; the neutron exposure in the neutron-rich regions is large enough for the Ne-22 to trigger a catastrophic r-process runaway in which the quantity of heavy elements doubles in much less than an expansion time due to fission cycling. A primordial abundance of r-process elements is predicted to appear as an excess of rare earth elements in extremely metal-poor stars.

Applegate, J. H.↗
NASA NTRS1972

The p-process in explosive nucleosynthesis.

The limiting conditions consistent with p-process synthesis in supernova envelopes are inferred from calculations of the appropriate rates of proton capture and neutron photodisintegration. Temperatures in excess of 2 x 10 to the 9th power K are required, for proton mass densities of the order of 100 g cm to the minus 3rd power, if significant production of p-process nuclei is to take place on a hydrodynamic time scale. It is concluded that these processes must be appropriate to very different stellar or supernova environments.

Truran, J. W.↗
NASA NTRS1990

Emission line nebulae in the Magellanic clouds

H II regions, planetary nebulae, and supernova remnants in the large and small Magellanic clouds have been favored targets among all galactic and extragalactic emission nebulae for study with the IUE satellite due to their known distances and low line of sight extinction. Several of the major results arising from successful IUE (International Ultraviolet Explorer) observations of these three types of emission nebulae in the clouds are presented. Carbon abundances in the H II regions from UV emission lines with implications regarding the chemical evolution of the clouds and stellar origins of these elements are discussed. The UV spectral characteristics of several evolved and young SNRs in the clouds with comparisons made to similar objects in the Galaxy and other nearby galaxies are discussed. CNO abundance variations among planetary nebulae and characteristics of their central stars with emphasis on the new insights provided regarding PN formation and C-N nucleosynthesis during the various AGB dredge-up phases are discussed.

Dufour, Reginald J.↗